Plum tree pest
Thank you for your question. Without seeing the actual insect that has inhabited the tree, a positive identification is impossible. However, here is one potential insect that causes this type of damage in plum trees: https://pnwhandbooks.org/insect/tree-fruit/plum-prune/plum-prune-pacific-flatheaded-borer
There is not much you can do with this branch, obviously, but following the suggestions in the article may help you save the remainder of the tree. Good luck!